1. Field of the Invention
The present invention relates to methods of manufacturing Nb.sub.3 Al based superconducting wire and coil, and more particularly, it relates to methods of manufacturing a wire and a coil for high field superconducting magnets which are applied to analytical NMR, nuclear fusion, power storage and the like.
2. Description of the Background Art
It is expected that an intermetallic compound superconducting material of Nb.sub.3 X, such as Nb.sub.3 Al, Nb.sub.3 Sn or Nb.sub.3 Ge, can be used in a high magnetic field in which an alloy superconducting material of NbTi or the like cannot be used. In particular, it has been confirmed by study of its physical properties that an Nb.sub.3 Al superconducting material has an excellent critical current property and excellent characteristics of the stress-strain effect under a high magnetic field.
At first, Nb.sub.3 Al could only be produced through a heat treatment at a high temperature of at least 1600.degree. C. for a short time. Therefore, it was regarded as impossible to produce wires of such a material in a high volume. However, it has recently been clarified that Nb.sub.3 Al can also be formed at a temperature of not more than 1600.degree. C., if a diffusion length between Nb and Al is extremely small. On the basis of such a recognition, there have been developed a powder metallurgy (PM) process, a composite working process such as a tube process and clad chip extrusion, and a jelly-roll process for manufacturing an Nb.sub.3 Al superconducting wire. Among such processes, the jelly-roll process is particularly useful for manufacturing a practical wire.
Nb.sub.3 Al, which is an intermetallic compound, is so mechanically fragile that the critical current is changed due to the strain and/or the stress applied to the material. In general, it is not easy to work an Nb.sub.3 Al superconducting wire, unlike an NbTi superconducting wire. In order to manufacture an Nb.sub.3 Al superconducting coil, it has typically been attempted to coil a wire which is prepared by composing Nb and Al in a state that does not form a compound and thereafter forming Nb.sub.3 Al in the coil by a heat treatment. However, it is difficult to perform a sufficient heat treatment on such a coil having a relatively large volume and relatively high thermal capacity for forming a compound at a high temperature in a short time. Because a copper or copper alloy matrix is used as a stabilizer in such a superconductor, further, it is necessary to perform the heat treatment at a temperature lower than the melting point of copper, i.e., not more than about 1083.degree. C.
Under such conditional restrictions, a process of forming Nb.sub.3 Al has been found, which involves directly heating a composite material prepared by overlapping an Al layer and an Nb layer with each other at 800.degree. to 850.degree. C. for 1 to 10 hours. The superconducting material prepared by such a process exhibits a higher current density than an Nb.sub.3 Sn superconducting material prepared by a conventional bronze process in a magnetic field of not more than about 12 T. In a higher magnetic field, however, this material cannot have a sufficient current density. While Nb.sub.3 Al itself is regarded as originally having a critical magnetic field of about 30 T, a superconducting material prepared by the aforementioned process has a critical magnetic field of about 20 T.